Human immunodeficiency virus (HIV) type 1 (HIV-1) and HIV type 2 (HIV-2) are genetically related, antigenically cross reactive, and share a common cellular receptor (CD4). See, Rosenburg and Fauci (1993) in Fundamental Immunology, Third Edition Paul (ed) Raven Press, Ltd., New York (Rosenburg and Fauci 1) and the references therein for an overview of HIV infection. HIV-1 infection is epidemic world wide, causing a variety of immune system-failure related phenomena commonly termed acquired immune deficiency syndrome (AIDS). HIV type 2 (HIV-2) has been isolated from both healthy individuals and patients with AIDS-like illnesses (Andreasson, et al. (1993) Aids 7, 989-93; Clavel, et al. (1986) Nature, 324, 691-695; Gao, et al. (1992) Nature 358, 495-9; Harrison, et al. (1991) Journal of Acquired Immune Deficiency Syndromes 4, 1155-60; Kanki, et al. (1992) American Journal of Epidemiology 136, 895-907; Kanki, et al. (1991) Aids Clinical Review 1991, 17-38; Romieu, et al. (1990) Journal of Acquired Immune Deficiency Syndromes 3, 220-30; Naucler, et al. (1993) International Journal of SYD and Aids 4, 217-21; Naucler, et al. (1991) Aids 5, 301-4). Although HIV-2 AIDS cases have been identified principally from West Africa, sporadic HIV-2 related AIDS cases have also been reported in the United States (O'Brien, et al. (1991) Aids 5, 85-8) and elsewhere. HIV-2 will likely become endemic in other regions over time, following routes of transmission similar to HIV-1 (Harrison, et al. (1991) Journal of Acquired Immune Deficiency Syndromes 4, 1155-60; Kanki, et al. (1992) American Journal of Epidemiology 136, 895-907; Romieu, et al. (1990) Journal of Acquired Immune Deficiency Syndromes 3, 220-30). Epidemiological studies suggest that HIV-2 produces human disease with lesser penetrance than HIV-1, and exhibits a considerably longer period of clinical latency (at least 25 years, and possibly longer, as opposed to less than a decade for HIV-1; see, Kanki, et al. (1991) Aids Clinical Review 1991, 17-38; Romieu, et al. (1990) Journal of Acquired Immune Deficiency Syndromes 3, 220-30, and Travers et al. (1995) Science 268: 1612-1615).
The molecular receptor for HIV is the surface glycoprotein CD4 found mainly on a subset of T cells, monocytes, macrophage and some brain cells. HIV has a lipid envelope with viral antigens that bind the CD4 receptor, causing fusion of the viral membrane and the target cell membrane and release of the HIV capsid into the cytosol. HIV causes death of these immune cells, thereby disabling the immune system and eventually causing death of the patient due to complications associated with a disabled immune system. HIV infection also spreads directly from cell to cell, without an intermediate viral stage. During cell-cell transfer of HIV, a large amount of viral glycoprotein is expressed on the surface of an infected cell, which binds CD4 receptors on uninfected cells, causing cellular fusion. This typically produces an abnormal multinucleate syncytial cell in which HIV is replicated and normal cell functions are suppressed.
Molecular analysis suggests that HIV-2 is more stable than HIV-1 in the human population, implying milder pathogenicity of the virus and introduction into the human population at a time earlier than HIV-1 (Clavel, et al. (1986) Nature, 324, 691-695; Gao, et al. (1992) Nature 358, 495-9; Naucler, et al. (1991) Aids 5, 301-4; O'Brien, et al. (1991) Aids 5, 85-8; Castro, et al (1990) Virology 178, 527-34; Kirchhoff, et al. (1990) Aids 4, 847-57; Kuhnel, et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 2383-2387; Kumar, et al. (1990) Journal of Virology 64, 890-901; Zagury, et al. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 5941-5945; Franchini, et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 2433-2437). Overlap of HIV-2 sequences with those of related simian immunodeficiency virus (SIV) isolates also provides evidence indicating that HIV-2 infection of humans originated through introduction of these primate lentiviruses through environmental or occupational (e.g., hunting, or cooking) exposure (Gao, et al (1992) Nature 358, 495-9).
Several HIV-2 isolates, including three molecular clones of HIV-2.sub.ROD, HIV-2.sub.SBL-ISY, and HIV-2.sub.UCl), have been reported to infect macaques (M. mulatta and M. nemestrina) or baboons (Franchini, et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 2433-2437; Barnett, et al. (1993) Journal of Virology 67, 1006-14; Boeri, et al. (1992) Journal of Virology 66, 4546-50; Castro, et al. (1991) Virology 184, 219-26; Franchini, et al. (1990) Journal of Virology 64, 4462-7; Putkonen, et al. (1990) Aids 4, 783-9; Putkonen, et al. (1991) Nature 352, 436-8). As human pathogens capable of infection of small primates, HIV-2 molecular clones provide attractive models for studies of AIDS pathogenesis, and for drug and vaccine development against HIV-1 and HIV-2.
Recently, HIV-2 was suggested as a possible vaccine candidate against the more virulent HIV-1 due to its long asymptomatic latency period, and its ability to protect against infection by HIV-1 (see, Travers et al. (1995) Science 268: 1612-1615 and related commentary by Cohen et al (1995) Science 268:1566). In the nine-year study by Travers et al. (id) of West African prostitutes infected with HIV-2 it was determined that infection with HIV-2 caused a 70% reduction in infection by HIV-1.
One notable characteristic of most HIV-2 isolates, in contrast to HIV-1, is their ability to readily infect primary monocyte-macrophages even after extensive passage on T-cell lines (Franchini, et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 2433-2437; Barnett, et al. (1993) Journal of Virology 67, 1006-14; Boeri, et al. (1992) Journal of Virology 66, 4546-50; Castro, et al. (1991) Virology 184, 219-26; Franchini, et al. (1990) Journal of Virology 64, 4462-7; Putkonen, et al. (1990) Aids 4, 783-9; Putkonen, et al. (1991) Nature 352, 436-81; Hattori, et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 8080-4). HIV-2, like SIV, encodes a vpx gene (Kappes, et al. (1991) Virology 184, 197-209; Marcon, et al. (1991) Journal of Virology 65, 3938-42), but lacks the vpu gene found in HIV-1. A consequence of the absence of vpu is that the HIV-2 envelope is not expressed as a bicistronic message. Other differences between HIV-1 and HIV-2 include differential sensitivity to non-nucleoside reverse transcriptase inhibitors (Bacolla, et al. (1993) Journal of Biological Chemistry 268, 16571-7), the variability and importance of the V3 region of envelope in neutralization (Bjorling, et al. (1994) Journal of Immunology 152, 1952-9; Chiodi, et al. (1993) Chemical Immunology 56, 61-77), the involvement of different transcriptional factors and T-cell signaling pathways in activation of the viral LTR (Hannibal, et al. (1993) Journal of Virology 67, 5035-40), and the specificity of the Tat and Rev transactivating proteins (Fenrick, et al. (1989) Journal of Virology 63, 5006-12; Malim, et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 8222-6).
The capacity to infect quiescent cells, which is not shared by oncoretroviruses or MoMLV-derived retroviral vectors, has spurred efforts to develop HIV-based gene therapy vectors. The goal of HIV-based vectors is stable transfer of genes to rarely dividing stem cells and post-mitotic cells in the hematopoietic, nervous, and other body systems. Progress has been made recently with vesicular stomatitis virus envelope glycoprotein (VSV-G)-pseudotyped HIV-1 vectors in this regard (Naldini et al. (1996) Science 272:263). Although high titers in the 10.sup.5 range were achieved, this system relies upon transient transfection to generate vector supernatants. Stable packaging cell lines were not developed. In addition, the vector is derived from HIV-1, a lentivirus with nearly uniform lethality in humans.
Other HIV vector systems have been studied. See, Akidna et al. (1996) J Virol 70:2581; Poznansky et al. (1991) J Virol 65:532; Parolin et al. (1994) Journal of Virology 68:3888; Richardson et al. (1995) Journal of General Virology 76:691; Buchschacher et al. (1992) Journal of Virology 66:2731; and Marlink et al. (1994) Science 265:1587. However, all are derived from HIV-1 and all evince a variety of limitations. Several use wild-type replication competent virus as the source of packaging proteins, and some represent simple pseudotyping of an env gene-mutated full-length provirus by VSV-G (i.e., no packaging construct, lines or vector lacking other structural genes). In general, two problems have been most prominent in this field: (1) titers, with the exception of Naldini et al. (1996; supra), have been exceedingly low (10.sup.1 -10.sup.2) (Poznansky, 1991 and Parolin, 1994, supra) or not reported (Adkina, 1996, supra) and (2) stable packaging lines have not been developed.
Accordingly, the isolation and development of non-pathogenic strains of HIV-2 as vaccines, in vitro diagnostic reagents, cell transduction and gene therapy vectors, and HIV-2 based packaging cell lines is needed. The present invention fulfills these and other needs.